In the prior art, reader apparatus for data carriers have typically employed a mounting arrangement designed to mount the transducer of the reader in one fixed relationship to the guide slot for the data carrier. For example, in a card reader designed to read two data tracks out of three possible tracks defined in a data card standard, the magnetic head mounting is designed to place the head and the two magnetic pickup head elements thereon in one fixed position relative to the bottom of the card slot. Thus a different mounting arrangement is required for reading tracks one and two together from that for reading tracks two and three together. The manufacture of a unit thus requires that the appropriate parts for one of these two mounting arrangements be obtained for assembly into the unit so that the required track reading will be achieved.